JP2013125874A - Laser element and optical record reproducing device with the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nitride-based semiconductor laser element capable of stably maintaining a single transverse mode even at a high output.SOLUTION: A current path from one electrode to the other electrode is formed between an N-electrode 10 and a P-electrode 22 of a laser element 1A, and an n-InGaN active layer 16 sandwiched between an n-AlGaN clad layer 14 and a p-AlGaN clad layer 19 exists in the current path. A central ridge structure 30 forming a current constriction region 33 at a portion between the P-electrode 22 and the n-InGaN active layer 16 is formed on the current path. A separation region 32 is provided outside the central ridge structure 30, and an outside ridge structure 31 is provided outside the separation region 32. An equivalent refractive index of the central ridge structure 30 is larger than that of the separation region 32, and the equivalent refractive index of the separation region 32 is smaller than that of the outside ridge structure 31.

Description

  The present invention relates to a laser element made of a nitride semiconductor and an optical recording / reproducing apparatus using the laser element as a light source.

  In the optical recording / reproducing apparatus, it is a problem to shorten the wavelength of the light source in order to increase the recording density. When an optical disk as a lens or a recording medium is formed of a plastic material, the wavelength of the light source is required to be around 400 nm. As such a short wavelength light source, a semiconductor laser element made of a nitride represented by gallium nitride is suitable. Examples of nitride-based semiconductor laser devices can be found in Patent Documents 1 to 3.

  The nitride-based semiconductor laser device described in Patent Document 1 has a ridge structure. This laser element has a light absorption part extending in the resonance direction of the light in the active layer at a distance of 0.3 μm or more from the lower part of the ridge structure, and this light absorption part allows spontaneous emission light that tends to have an adverse effect on the use of laser light. Reduction is being achieved.

  The nitride-based semiconductor laser device described in Patent Document 2 has a striped ridge structure. The laser element is provided with a recess so as to be in contact with the emission end surface, and the side surface of the recess is a rough surface. Since the leaked light from the ridge stripe is not reflected on the side surface of the recess, it is not subjected to multiple reflection with the highly reflective surface, and noise is prevented from occurring in the current-light output characteristics.

  The nitride-based semiconductor laser device described in Patent Document 3 includes a ridge stripe. A GaN layer doped with impurities is formed on at least a part of the exposed surface formed by the ridge stripe, and this GaN layer serves as an absorption layer so that the far-field image has a single-peak Gaussian shape without ripples. I have to.

Japanese Patent No. 3989244 JP 2006-287137 A JP 2006-216731 A

  A nitride-based semiconductor laser element used as a light source of an optical recording / reproducing apparatus needs to operate in a single transverse mode because it is focused by a lens. However, when the output increases, the single transverse mode collapses and there is a tendency to change to the multiple transverse mode. This appears as a bend in the current-light output characteristic, and the bend is called an IL characteristic kink. It is necessary to avoid as much as possible the multi-lateral mode that makes it difficult to collect light with a lens.

  One means of maintaining a single transverse mode is to reduce the thickness of the waveguide layer that guides light. However, narrowing the waveguide layer thickness in a semiconductor laser device having a ridge structure is synonymous with narrowing the current confinement region, and a higher driving voltage is required.

  The maintenance of the single transverse mode becomes more difficult as the oscillation wavelength becomes shorter and the material refractive index becomes smaller. Semiconductor laser elements such as GaAs, which are currently widely used as light sources for optical recording and reproducing devices, have oscillation wavelengths of about 660 nm and 780 nm and a material refractive index of about 3.5, whereas nitride-based semiconductor laser elements Has an oscillation wavelength of 405 nm and a material refractive index of about 2.5. That is, it is difficult for a nitride semiconductor laser element to maintain a single transverse mode as compared with a semiconductor laser element such as GaAs.

  If the technique of reducing the waveguide layer thickness is applied to a nitride semiconductor laser element, the current confinement region needs to be narrower than that of a semiconductor laser element such as GaAs, and a higher operating voltage is required. Since it is better that the operating voltage of the laser element is low, it is difficult to introduce this method.

According to the structure of the nitride-based semiconductor laser device described in Patent Document 1, since the light absorbing portion is provided outside the ridge structure, the oscillation of the higher-order transverse mode that is the cause of the multilateral mode is suppressed. The However, since the laser beam operating in the single transverse mode is absorbed by the light absorption part, the light extraction efficiency is lowered and the oscillation threshold current value is increased. In nitride-based semiconductor laser devices having a light absorption part, in order to reduce the absorption of laser light operating in a single transverse mode as much as possible, a process accuracy of the order of 1 × 10 ⁻¹ μm is required. Cause a decline.

  The present invention has been made in view of the above points, and does not cause problems such as a decrease in light extraction efficiency, an increase in oscillation threshold current value, and an increase in drive voltage, and a stable single transverse mode can be achieved even at high output. An object of the present invention is to provide a nitride semiconductor laser device that can be maintained. It is another object of the present invention to provide a high-performance optical recording / reproducing apparatus including such a nitride semiconductor laser element as a light source.

  In order to achieve the above object, a laser device according to the present invention has at least an n-type nitride semiconductor layer, an active layer, and a p-type nitride semiconductor layer on a substrate, and has a central ridge structure serving as a striped waveguide. A nitride-based semiconductor laser device provided, comprising a separation region outside the central ridge structure, and an outer ridge structure outside the separation region, wherein the equivalent refractive index of the central ridge structure is the separation The equivalent refractive index of the region is larger, and the equivalent refractive index of the separated region is smaller than the equivalent refractive index of the outer ridge structure.

  In the laser device having the above structure, it is preferable that a distance between the central ridge structure and the outer ridge structure is 1.0 μm or less across the separation region.

  In the laser device having the above structure, the material constituting the separation region and the outer ridge structure is made of a material having an absorption coefficient of 0.1 or less with respect to the laser emission wavelength, excluding the metal of the electrode material and the material in the active layer. It is preferred that

  In the laser element having the above-described configuration, it is preferable that the separation region is formed of any one of a metal, a semiconductor, a dielectric, an insulator, and a space.

In the laser device having the above configuration, when the separation region is formed of an insulator, the insulator includes SiO ₂ , TiO ₂ , Al ₂ O ₃ , Ta ₂ O ₅ , ZrO ₂ , In ₂ O ₃ , Nd _2. One of O ₃ , Sb ₂ O ₃ , CeO ₂ , ZnS, AlN, AlON, Si ₃ N ₄ and Bi ₂ O ₃ is preferably selected.

  In the laser element having the above-described configuration, it is preferable that the separation region and the outer ridge structure gradually increase as the equivalent refractive index moves away from the central ridge structure.

  In the laser device having the above configuration, the separation region includes at least a separation region first layer and a separation region second layer above the p-type nitride semiconductor layer, and the separation region first layer is closer to the active layer. The second layer of the separation region is disposed on a side far from the active layer, a layer forming an outer ridge structure is disposed outside the separation region, and a refractive index with respect to a laser emission wavelength forms the outer ridge structure. It is preferable that the layer is larger than the first layer in the separation region.

  In the laser device having the above structure, the width of the central ridge structure is preferably 0.5 μm or more and 5 μm or less.

  Further, the present invention is an optical recording / reproducing apparatus including the laser element having the above-described configuration as a light source.

  According to the configuration of the present invention, a fundamental transverse mode having a unimodal light output distribution can be oscillated immediately below the current confinement region sandwiched between the central ridge structures, and the gain can be sufficient for laser oscillation. In a high-order transverse mode with a bimodal light output distribution, the position of the peak shifts to the outside of the current confinement region, and light is emitted outward of the current confinement region, leading to laser oscillation. Can't get gain. For this reason, a stable single transverse mode operation can be maintained without shifting to a high-order transverse mode even in a high output region.

1 is a schematic cross-sectional view of a laser device according to a first embodiment of the present invention and a graph of an equivalent refractive index in the laser device. It is the typical sectional view of the laser device concerning a 2nd embodiment of the present invention, and the graph of the equivalent refractive index in the laser device concerned. It is a typical sectional view of a laser device concerning a 3rd embodiment of the present invention, and a graph of an equivalent refractive index in the laser device. It is a typical sectional view of a laser device concerning a 4th embodiment of the present invention, and a graph of an equivalent refractive index in the laser device. It is a typical sectional view of a laser device concerning a 5th embodiment of the present invention, and a graph of an equivalent refractive index in the laser device. It is a typical sectional view of a laser device concerning a 6th embodiment of the present invention, and a graph of an equivalent refractive index in the laser device. It is a typical sectional view of a laser device concerning a 7th embodiment of the present invention, and a graph of an equivalent refractive index in the laser device concerned. It is a typical sectional view of a laser device concerning an 8th embodiment of the present invention, and a graph of an equivalent refractive index in the laser device. It is an operation | movement principle figure of this invention.

  FIG. 1 shows a nitride semiconductor laser element (hereinafter simply referred to as “laser element”) 1A according to the first embodiment. The laser element 1A includes an N electrode 10, an n-GaN substrate 11, an n-GaN layer 12, an n-InGaN crack prevention layer 13, an n-AlGaN cladding layer 14, an n-GaN guide layer 15, and an n-InGaN in this order from the bottom. The active layer 16, the p-AlGaN barrier layer 17, the p-GaN guide layer 18, the p-AlGaN cladding layer 19, the p-GaN contact layer 20, the insulating film 21, and the P electrode 22 are stacked.

  From the n-GaN layer 12 to the p-GaN contact layer 20 is a nitride-based semiconductor. Among them, the n-GaN layer 12 to the n-InGaN active layer 16 are n-type nitride semiconductor layers and active layers, and the p-AlGaN barrier layer 17 to the p-GaN contact layer 20 are p-type nitride semiconductors. Is a layer.

  Three pairs of striped ridge structures are formed from the upper part of the p-AlGaN cladding layer 19 to the p-GaN contact layer 20. That is, a pair of ridge structures 30 are formed in the center, and a pair of ridge structures 31 are formed on the left and right sides thereof. Hereinafter, the ridge structure 30 is referred to as a central ridge structure 30 and the ridge structure 31 is referred to as an outer ridge structure. In the central ridge structure 30 and the outer ridge structure 31, only a pair of ridge structures located on the left side in the drawing is given a reference numeral.

  The end of the central ridge structure 30 and the end of the outer ridge structure 31 are separated by the width D of the separation region 32. The “edge of the ridge structure” is defined as “a position outside the ridge structure region and as close as possible to the side surface of the ridge structure”. The value of the width D of the separation region 32 is 1.0 μm or less, and the central ridge structure 30 and the outer ridge structure 31 are in a close positional relationship even though they are separated.

  The insulating film 21 covers the side surface of the central ridge structure 30, the side surface and the upper portion of the outer ridge structure 31, and the upper surface of the p-AlGaN cladding layer 19 located on the side of the outer ridge structure 31. In the p-GaN contact layer 20, a portion sandwiched between the central ridge structures 30 is exposed upward. The N electrode 10 covers substantially the entire lower surface of the substrate 11, and the P electrode 22 covers the exposed portion of the p-GaN contact layer 20 and the insulating film 21.

  The laser element 1A is manufactured as follows. First, an n-GaN layer 12, an n-InGaN crack prevention layer 13, and an n-AlGaN cladding layer 14 are formed on the n-GaN substrate 11 by using a crystal growth method such as a commonly used MOCVD (metal organic chemical vapor deposition) method. The n-GaN guide layer 15, the n-InGaN active layer 16, the p-AlGaN barrier layer 17, the p-GaN guide layer 18, the p-AlGaN cladding layer 19, and the p-GaN contact layer 20 are stacked.

  Subsequently, a central ridge structure 30 and an outer ridge structure 31 are formed. The central ridge structure and the outer ridge structure 31 are formed by removing part of the p-GaN contact layer 20 to the middle of the p-AlGaN cladding layer 19. The removal of the p-GaN contact layer 20 and the p-AlGaN cladding layer 19 can be performed by an ICP (inductively coupled plasma) etching apparatus using photolithography.

  By processing with the ICP etching apparatus, the central ridge structure 30 and the outer ridge structure 31 can be simultaneously formed with high accuracy in one process so that the width D of the separation region 32 is 1.0 μm or less. The width of the central ridge structure is 0.5 μm or more and 5 μm or less. One pair of outer ridge structures 31 are provided on both sides of the central ridge structure 30, but only one outer ridge structure 31 may be provided at a position facing the central ridge structure 30.

Subsequently, an insulating film 21 made of SiO ₂ is formed. The insulating film 21 is formed by a normal sputtering method or the like. In FIG. 1, the insulating film 21 is formed in such a manner that the upper portion of the p-AlGaN cladding layer 19 is completely filled with the separation region 32 between the central ridge structure 30 and the outer ridge structure 31.

  After the formation of the insulating film 21, the upper part of the portion sandwiched between the pair of central ridge structures 30 is opened by photolithography, and the insulating film 21 at that portion is removed by wet etching. Thereafter, the N electrode 10 and the P electrode 22 are formed by a normal method, and the wafer is cleaved and divided to obtain a chip of the semiconductor laser element 1A. A reflective film and a protective film are formed as necessary on the cleavage end face perpendicular to the ridge stripe of the ridge structure.

  A current flows between the P electrode 22 and the N electrode 10. A location where current flows is a current path. The current path is restricted by the insulating film 21, and current is injected only from the exposed portion of the p-GaN contact layer 20 where the insulating film 21 does not exist. The width of current injection into the n-InGaN active layer 16 is substantially equal to the distance between the lower ends of the central ridge structure 30. That is, a portion of the current path sandwiched by the central ridge structure 30 becomes the current confinement region 33. The width of the current confinement region 33 is 0.5 μm or more and 5 μm or less.

  Of the n-InGaN active layer 16, a portion located immediately below the current confinement region 33 is a portion involved in the generation of laser light. The light resonates inside the n-InGaN active layer 16, and the generated laser light is emitted from the front end surface of the paper or from both the front and back end surfaces. Since the current confinement region 33 is formed, the width of the laser beam is narrowed, the density of the current flowing through the n-InGaN active layer 16 is increased, and the driving power necessary for laser oscillation can be reduced. The separation region 32 and the outer ridge structure 31 are made of a material having an absorption coefficient of 0.1 or less with respect to the laser emission wavelength. However, the electrode and the active layer may be made of a material (a metal in the case of an electrode) having an absorption coefficient of 0.1 or more with respect to the laser emission wavelength.

The fundamental mode of laser oscillation is a single transverse mode. Even if the central ridge structure 30 is not optimized to be a single transverse mode, it basically oscillates in a single transverse mode. However,
When the amount of injected current (hereinafter referred to as “injected current”) increases, the transverse mode becomes bimodal and multimode (multilateral mode). This is known as a kink phenomenon in current-light output characteristics. The mechanism of occurrence of such a phenomenon will be described below.

  In the region where the injection current amount is small, the unimodal carrier density distribution in the n-InGaN active layer 16 and the unimodal fundamental transverse mode light intensity distribution are in good agreement. The fundamental transverse mode has a larger gain than the higher-order transverse mode having, and operates in the fundamental transverse mode.

  When the amount of injected current increases, the central portion of the n-InGaN active layer 16 immediately below the current confinement region 33 has a higher light intensity than the outer side, so that the amount of carrier consumption increases and the carrier density distribution becomes concave. This phenomenon is called spatial hole burning. Thus, when the carrier density distribution becomes concave, it is difficult to obtain a gain in the fundamental transverse mode. On the other hand, the higher-order transverse mode having a bimodal light intensity distribution is less likely to reduce the gain than the fundamental transverse mode. As this tendency progresses, the higher-order transverse mode gain exceeds the fundamental transverse mode gain, and higher-order transverse mode oscillation occurs, resulting in a kink phenomenon.

  As described above, the occurrence of the kink phenomenon is caused by oscillation in a high-order transverse mode. Therefore, in order to avoid the occurrence of the kink phenomenon, it is only necessary to reduce the gain of the high-order transverse mode. Therefore, in the present invention, the oscillation in the fundamental transverse mode that is a unimodal light intensity distribution does not act on the outside of the current confinement region 33 so as to reduce the gain, and the bimodal light intensity distribution and For the oscillation in the higher order transverse mode, a structure that acts to reduce the gain is formed to reduce the gain in the higher order transverse mode.

  In the laser element 1 </ b> A, the separation region 32 formed outside the central ridge structure 30 and the outer ridge structure 31 constitute a structure that reduces the gain of the high-order transverse mode.

  The fundamental transverse mode having a unimodal strong light intensity distribution immediately below the current confinement region 33 stably obtains gain by the injection current from the P electrode 22 through the current confinement region 33. Since the current path is narrowed, the width of the generated laser beam is narrowed and the density of the current flowing through the n-InGaN active layer 16 is increased, so that the driving power necessary for laser oscillation can be reduced.

  On the other hand, in the high-order transverse mode having a bimodal light output distribution, the outer ridge structure 31 is provided on both sides of the central ridge structure 30 in a close positional relationship of 1.0 μm or less across the width D of the separation region 32. As a result, the bimodal light intensity distribution spreads in a direction parallel to the spread of the n-InGaN active layer 16. As described above, the fundamental transverse mode having a strong unimodal light intensity distribution immediately below the current confinement region 33 can obtain a stable gain by the injection current from the upper part of the central ridge structure 30, whereas the central ridge structure. In the high-order transverse mode having a strong light intensity distribution outside 30, no current is injected from the upper part of the outer ridge structure 31, so that it is difficult to obtain a gain. For this reason, even if the amount of injected current increases, the gain of the higher-order transverse mode does not exceed the gain of the fundamental transverse mode, and the higher-order transverse mode does not oscillate. That is, the basic transverse mode operation is maintained even when the injection current amount reaches a high level.

  When the width D of the separation region 32 is increased, the effect of avoiding the kink phenomenon is lost. Experiments and calculations show that the value of the width D of the separation region 32 that does not affect the bimodal light intensity distribution in the high-order transverse mode, that is, the effect of suppressing kink cannot be obtained is more than 1.0 μm. ing.

  A graph of equivalent refractive index is attached at the top of FIG. The nitride-based semiconductor laser device of the present invention has virtual regions I to VII partitioned by a plane perpendicular to the laminated surface of the nitride-based semiconductor layers. The region I to the region VII are virtually partitioned by the interfaces of the respective structures such as the central ridge structure 30, the separation region 32, and the outer ridge structure 31. The equivalent refractive index is calculated from region I to region VII for each region as follows.

  The regions are divided into a large number in the direction parallel to the surface of the n-InGaN active layer 16, that is, in the horizontal direction so that the layer structure in the direction perpendicular to the surface of the n-InGaN active layer 16 is the same. Using each region having the same vertical layer structure as a simple multilayer slab waveguide, the propagation coefficient β of the fundamental transverse mode is obtained from the eigenvalue equation derived from the wave equation. The value obtained by dividing the propagation coefficient β by the wave number k0 in vacuum is the equivalent refractive index of the waveguide mode in the direction perpendicular to the surface of the n-InGaN active layer 16. In other words, the equivalent refractive index of the present invention is, for example, the thickness, refractive index, and the like of semiconductor layers, insulating layers, metal layers, and spaces in the direction perpendicular to the nitride semiconductor layer stack surface in the regions I to VII in FIG. The refractive index for each region derived from the rate is used.

  When the layer structure changes continuously in the horizontal direction, the region is divided in units of 0.1 μm in the horizontal direction, and the refractive index, the maximum value of the layer thickness, and the average value of the minimum values in each divided region are calculated. The refractive index and the layer thickness of the divided area are calculated.

  In FIG. 1, when viewing the graph of the equivalent refractive index of the laser device 1 </ b> A, the equivalent refractive index of the separation region 32 is smaller than the equivalent refractive index of the central ridge structure 30 and the outer ridge structure 31. Such an equivalent refractive index distribution has the following effects on laser oscillation in a single transverse mode and laser oscillation in a higher-order transverse mode.

  FIG. 9 shows three horizontally long ellipses together with the graph shape of FIG. One ellipse (shaded) drawn above symbolizes a single transverse mode. Two ellipses drawn below (with horizontal hatching) symbolize higher-order transverse modes.

The single transverse mode, which is the fundamental mode, has a unimodal light output distribution peak between locations where the equivalent refractive index is lowered. For a single transverse mode, the gain can be sufficient for lasing, but a higher order transverse mode with a bimodal optical power distribution has a peak position outward from where the equivalent refractive index has decreased. Shifts and the light is emitted outward, and a gain sufficient to cause laser oscillation cannot be obtained. For this reason, a stable single transverse mode operation can be maintained without shifting to a high-order transverse mode even in a high output region.

  Next, the structure of the laser device after the second embodiment will be described with reference to FIGS. In any embodiment, the same or functionally common components as those in the first embodiment are denoted by the same reference numerals as those used in the first embodiment, and the description thereof is omitted.

  A laser element 1B according to the second embodiment shown in FIG. 2 is a modification of the laser element 1A according to the first embodiment. That is, in the laser device 1A, the outer ridge structures 31 on both sides of the central ridge structure 30 are ridge structures having a pair of side surfaces, but in the laser device 1B, the side surfaces of the outer ridge structure 31 facing the central ridge structure 30. The side that does not face the central ridge structure 30 is omitted.

  The outer ridge structure 31 of the laser device 1B of the second embodiment is formed by etching using a photolithographic mask pattern different from the first embodiment. In the laser device 1B of the second embodiment, it is difficult to control the bimodal light in a limited region in both side surfaces of the outer ridge structure 31, but the ridge is different from the laser device 1A of the first embodiment. There is an advantage that the effort of forming the structure is reduced. In other words, even in the side surface of the outer ridge structure 31 that faces the central ridge structure 30, the bimodal high-order transverse mode has an effect of radiating light to the outer ridge structure 31.

  The laser device 1C of the third embodiment shown in FIG. 3 differs from the laser device 1A of the first embodiment in the following points. That is, in the laser element 1 </ b> C, the separation region first layer 34 is formed outside the current confinement region 33, at a position between the central ridge structure 30 and the outer ridge structure 31. The separation region first layer 34 starts from the end of the central ridge structure 30 and has a width of 1.0 μm or less. The separation region first layer 34 exists only in the p-AlGaN cladding layer 19 or the p-AlGaN guide layer 18 and does not have a depth reaching the p-AlGaN barrier layer 17.

  After the ridge etching step, the separation region first layer 34 can be formed by using a photolithography method again and laminating a material by sputtering or the like in the separation region 32 and the exposed portion of the p-AlGaN cladding layer 19. Further, when the separation region first layer 34 is set to a space or a vacuum, the separation region 32 can be formed without being completely embedded by adjusting the film forming conditions of the insulating film 21.

  By forming the separation region first layer 34, the equivalent refractive index difference of the separation region 32 with respect to the central ridge structure 30 and the outer ridge structure 31 is more accurate or more flexible than in the first and second embodiments. Can be a thing.

  The separation region first layer 34 is composed of any one of a semiconductor, a dielectric, an insulator, and a space.

  When the separation region first layer 34 is constituted by a space, the space may be a vacuum or may be filled with a gas such as air. When the separation region first layer 34 is formed by the air layer, the equivalent refractive index step can be made larger than in the case where the separation region first layer 34 is formed by the dielectric, so that the oscillation of the high-order transverse mode can be achieved. This is more desirable in terms of suppression.

  When the separation region first layer 34 is made of a dielectric, it is desirable that the refractive index of the dielectric be as small as possible than the refractive index of the p-AlGaN cladding layer 19.

When the separation region first layer 34 is made of an insulator, the insulator includes TiO ₂ , Al ₂ O ₃ , Ta ₂ O ₅ , ZrO ₂ , In ₂ O ₃ , Nd ₂ O ₃ , and Sb ₂ O _{3. , CeO 2, ZnS, AlN,} AlON, or the Si ₃ N ₄ and Bi ₂ O ₃ is selected.

  In the laser device 1D of the fourth embodiment shown in FIG. 4, the boundary between the separation region 32 and the outer ridge structure 31 is not clear. Instead, the thickness of the p-AlGaN cladding layer 19 gradually increases from the end of the central ridge structure 30 toward the outside. The region 35 outside the end of the central ridge structure 30, that is, the region 35 outside the current confinement region 33 is a structure in which high-order transverse mode light having a bimodal light output distribution is emitted outward.

  In the ridge etching process, the region 35 is different from the first embodiment in the shape of the photomask, the resist type, the resist formation conditions, and the exposure conditions, or the etching rate conditions are changed so that the portion close to the central ridge structure 30 is preferential. The thickness of the p-AlGaN cladding layer 19 is gradually increased by etching.

  The equivalent refractive index of region 35 increases gradually as it moves away from the central ridge structure 30. Due to the gradient of the equivalent refractive index, the phenomenon shown in the operation principle diagram of FIG. 9 occurs, and the region 35 decreases the gain of the high-order transverse mode.

  A laser element 1E according to the fifth embodiment shown in FIG. 5 is a modification of the laser element 1D according to the fourth embodiment. That is, in the laser element 1D, the thickness of the p-AlGaN cladding layer 19 is linear, but in the laser element 1E, it is curved. That is, the gradient of increasing thickness is steep near the central ridge structure 30, and the gradient of increasing thickness is gentle as the distance from the central ridge structure 30 increases.

  In the ridge etching process, the region 35 is different from the first embodiment in the shape of the photomask, the resist type, the resist formation conditions, and the exposure conditions, or the etching rate conditions are changed to give priority to a portion close to the central ridge structure 30. The thickness of the p-AlGaN cladding layer 19 is gradually increased by etching. With this configuration, the distance D can be further reduced.

The laser device 1F of the sixth embodiment shown in FIG. 6 also forms the outer ridge structure 31 with a material different from that of the p-AlGaN cladding layer 19 and the p-GaN contact layer 20. The layer 36 constituting the outer ridge structure 31 is TiO ₂ , Al ₂ O ₃ , Ta ₂ O ₅ , ZrO ₂ , In ₂ O ₃ , Nd ₂ O ₃ , Sb ₂ O ₃ , CeO ₂ , ZnS, AlN, Si _3. It is desirable that it is either N ₄ or Bi ₂ O ₃ . The layer 36 forming the outer ridge structure can be formed by sputtering or the like using a photolithography method after the ridge etching process.

  The equivalent refractive index difference between the central ridge structure 30 and the outer ridge structure 31 is the same in the first embodiment, but by forming the layer 36 forming the outer ridge structure 31, the central ridge structure 30 and the outer ridge structure 31 are formed. The equivalent refractive index difference of the separation region with respect to 31 can be formed with higher accuracy or higher degree of freedom than in the first and second embodiments. Further, the equivalent refractive index difference between the central ridge structure 30 and the outer ridge structure 31 can also be set arbitrarily.

The layer 36 forming the outer ridge structure 31 includes SiO ₂ , TiO ₂ , Al ₂ O ₃ , Ta ₂ O ₅ , ZrO ₂ , In ₂ O ₃ , Nd ₂ O ₃ , Sb ₂ O ₃ , CeO ₂ , ZnS, and AlN. , Si ₃ N ₄ and Bi ₂ O ₃ , or a semiconductor such as GaN or AlGaN.

  A laser device 1G of the seventh embodiment shown in FIG. 7 is a modification of the laser device 1F of the sixth embodiment. In the laser element 1G, a layer 37 forming a separation region is provided between the layer 36 forming the central ridge structure 30 and the outer ridge structure 31. The layer 36 forming the outer ridge structure 31 and the layer 37 forming the separation region 37 are selectively formed on the exposed surface of the p-AlGaN cladding layer 19 etched by a photolithography method or the like after the ridge etching process. The layer 36 forming the outer ridge structure and the layer 37 forming the separation region are made of different materials.

Layer 37 constituting the distance _{is, SiO 2, TiO 2, Al} 2 O 3, Ta 2 O 5, ZrO 2, In 2 O 3, Nd 2 O 3, Sb 2 O 3, CeO 2, ZnS, AlN, Si ₃ N ₄ and Bi ₂ O ₃ are preferably an insulator or space.

  With the above configuration, the equivalent refractive index difference between the central ridge structure 30 and the outer ridge structure 31 has the same value in the first embodiment, but the layer 36 forming the outer ridge structure 31 and the layer 37 forming the separation region are formed. Thus, the equivalent refractive index difference in the separation region with respect to the central ridge structure 30 and the outer ridge structure 31 can be formed with higher accuracy or higher degree of freedom than in the first and second embodiments. Further, the equivalent refractive index difference between the central ridge structure 30 and the outer ridge structure 31 can also be set arbitrarily.

  A laser element 1H according to the eighth embodiment shown in FIG. 8 is a modification of the laser element 1G according to the seventh embodiment. In the laser element 1 </ b> H, a layer 38 forming a separation region is provided between the central ridge structure 30 and the P electrode 22. The layer 38 forming the separation region is made of a different material from the layer 37 forming the separation region. The layer 37 forming the separation region is referred to as a “separation region first layer 37”, and the layer 38 forming the separation region is referred to as a “separation region second layer 38”. The separation region second layer 38 and the layer 36 forming the outer ridge structure 31 may be made of different materials, but are preferably made of the same material for ease of process.

  With the above configuration, the equivalent refractive index difference between the central ridge structure 30 and the outer ridge structure 31 has the same value in the first embodiment, but the layer 36, the separation region first layer 37, and the separation region that form the outer ridge structure 31. By forming the second layer 38, the equivalent refractive index difference in the separation region with respect to the central ridge structure 30 and the outer ridge structure 31 is more accurate or more flexible than the first and second embodiments. Can be formed. The refractive index with respect to the laser emission wavelength is formed so that the layer 36 forming the outer ridge structure 31 is larger than the first layer 37 in the separation region. The equivalent refractive index difference between the central ridge structure 30 and the outer ridge structure 31 can also be set arbitrarily.

  The laser elements 1A, 1B, 1C, 1D, 1E, 1F, 1G, and 1H can be used as a light source of an optical recording / reproducing apparatus that guides light from a light source to an optical recording medium to record and reproduce information.

  Although the embodiments of the present invention have been described above, the scope of the present invention is not limited to these embodiments, and various modifications can be made without departing from the spirit of the invention.

  The present invention can be widely used for a laser element made of a nitride semiconductor and an optical recording / reproducing apparatus using the laser element as a light source.

1A, 1B, 1C, 1D, 1E, 1F, 1G, 1H Nitride semiconductor laser element 10 N electrode 11 n-GaN substrate 12 n-GaN layer 13 n-InGaN crack prevention layer 14 n-AlGaN cladding layer 15 n-GaN Guide layer 16 n-InGaN active layer 17 p-AlGaN barrier layer 18 p-GaN guide layer 19 p-AlGaN cladding layer 20 p-GaN contact layer 21 insulating film 22 P electrode 30 central ridge structure 31 outer ridge structure 32 spaced region 33 Current confinement region

Claims (9)

A nitride-based semiconductor laser device having at least an n-type nitride semiconductor layer, an active layer, and a p-type nitride semiconductor layer on a substrate and provided with a central ridge structure serving as a striped waveguide,
A separation region is provided outside the central ridge structure,
An outer ridge structure is provided outside the spacing region,
The equivalent refractive index of the central ridge structure is greater than the equivalent refractive index of the spaced region,
The nitride-based semiconductor laser device, wherein an equivalent refractive index of the separation region is smaller than an equivalent refractive index of the outer ridge structure.   2. The laser device according to claim 1, wherein a distance between the central ridge structure and the outer ridge structure is 1.0 μm or less across the separation region.   The material constituting the spaced region and the outer ridge structure is composed of a material having an absorption coefficient of 0.1 or less with respect to the laser emission wavelength, excluding the metal of the electrode material and the material in the active layer. The laser device according to claim 1 or 2.   The laser element according to any one of claims 1 to 3, wherein the separation region is formed of any one of a metal, a semiconductor, a dielectric, an insulator, and a space. When the separation region is formed of an insulator, the insulator includes SiO ₂ , TiO ₂ , Al ₂ O ₃ , Ta ₂ O ₅ , ZrO ₂ , In ₂ O ₃ , Nd ₂ O ₃ , Sb ₂ O _3. 5. The laser element according to claim 4, wherein any one of CeO ₂ , ZnS, AlN, AlON, Si ₃ N ₄ and Bi ₂ O ₃ is selected.   6. The laser device according to claim 1, wherein the separation region and the outer ridge structure gradually increase in the equivalent refractive index as the distance from the central ridge structure increases.   The separation region includes at least a separation region first layer and a separation region second layer above the p-type nitride semiconductor layer, and the separation region first layer is disposed on a side closer to the active layer. Two layers are arranged on the side far from the active layer, a layer forming an outer ridge structure is arranged outside the spaced region, and a refractive index with respect to a laser emission wavelength is higher in the layer forming the outer ridge structure. 6. The laser element according to claim 1, wherein the laser element is larger than the first layer.   The laser element according to any one of claims 1 to 7, wherein a width of the central ridge structure is not less than 0.5 µm and not more than 5 µm.   An optical recording / reproducing apparatus for recording and reproducing information by guiding light from a light source to an optical recording medium, comprising the laser element according to any one of claims 1 to 8 as a light source. apparatus.

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